Before commencing excavation or other work where electrical cables, fibre optic cables or other utilities ducts or pipes are buried, it is important to determine the location of such buried cables or pipes to ensure that they are not damaged during the work. Once a buried utility is located the depth of the utility can be calculated to determine a safe excavation depth.
Current carrying conductors emit electromagnetic radiation which can be detected by an electrical antenna. If fibre optic cables or non-metallic utilities ducts or pipes are fitted with a small electrical tracer line, an alternating electrical current can be induced in the tracer line which in turn radiates electromagnetic radiation. It is known to use detectors to detect the electromagnetic field emitted by conductors carrying alternating current.
One type of such detector works in one of two modes, namely ‘active’ or ‘passive’ modes. Each mode has its own frequency band or bands of detection.
In passive mode, the detector detects ambient magnetic fields, for example those produced by a conductor carrying an AC mains power supply at 50/60 Hz and very low frequency (VLF) signals originating from VLF long wave transmitters.
In active mode a signal generator/transmitter is used to produce an alternating test signal in the conductor in accordance with one of three mechanisms. If the transmitter can be directly connected to the conductor then an alternating test signal of known frequency waveform and modulation is applied directly to the conductor.
If the conductor is accessible but direct connection is not feasible, for example where the conductor is carrying live mains power, a clamp can be used to apply the transmitter test signal to the conductor. The clamp is typically comprised of a split toroidal magnetic core which carries a primary winding magnetising the core with the alternating transmitter signal. An alternating signal flowing in the winding produces an electromagnetic signal in the conductor similar in operation to a transformer.
Where access to the conductor is not possible, the signal transmitter produces an alternating electromagnetic field by use of a strong induction loop. If the transmitter is placed near to the buried conductor then the electromagnetic field induces a current in a nearby buried conductor.
In all three mechanisms of stimulating a test signal in the buried conductor in the active mode, the buried conductor radiates the signal produced by the signal transmitter and the radiated signal can be detected by a detector.
A number of factors must be considered when using the active mode. As the transmitter is conventionally powered by on-board batteries it is important to efficiently generate the test signal whilst conserving the power expended by the transmitter as much as possible so as to prolong the battery life of the transmitter. Therefore the power of a signal output from the transmitter should be minimised to reduce battery consumption. In addition, a high power signal can couple to unwanted lines and spread over the lines, making it difficult to detect the target buried conductor.
The transmitter can be configured to transmit an alternating test signal at a number of frequencies and waveform types. The choice of frequency depends on a number of factors, for example the ease of inducing the test signal into the buried conductor and interference from ambient signals.
Regarding the choice of frequency of the alternating test signal, a high frequency signal is typically used for a high resistance line or a small insulated telecoms line, although that signal decreases more rapidly with distance along the conductor than for a lower frequency. A medium frequency signal is typically used for mains power supply cables and continuous metal pipes and a low frequency signal is used for long distance tracing where a defined termination is used (earth).
A problem with conventional signal transmitters is that the transmitter performs poorly in response to a change in the load, which can lead to damage to the transmitter. Although conventional signal transmitters comprise a basic feedback loop to stabilise the signal output from the transmitter, the control law used does not allow the transmitter to react quickly to changes in the load. For example, sudden disconnection of the load may not be expediently detected by the transmitter, resulting in driving the amplifiers too hard and potentially damaging the transmitter. In addition, the feedback loop can detect and process ambient signals and inefficiently drive amplifiers in the transmitter based on the ambient signals.